1. Field of the Invention
The present invention is related to a method for manufacturing lead frames and a lead frame material including an intermediate layer and a top layer for improving the reliability of wire bonding such as anti-corrosion, anti-oxidation, solderability, and less bending cracking in the semiconductor device.
2. Related Art
One of the popular approaches to electrically interconnect an IC unit to circuitry external of the IC device takes the form of a lead frame made of copper or copper alloy. The lead frame includes die pads on which semiconductor chips are fixed. The inner leads are connected to semiconductor chips and outer leads are connected to printed circuit board. However, the surface of the lead frame is easily oxidized by contact with air. The oxidation of lead frame will cause a drop in bonding strength between the inner lead and the bonding wires which connect bonding pads of the semiconductor chips to the corresponding inner leads of the lead frames. Conventional means for preventing the drop in bonding strength by the oxidation is to form a layer of SnPb or Ag on the surface of lead frames.
A pre-plated frame (PPF) is another technique to avoid the oxidation of copper in air. In this technique, the copper base is blanket plated with a protective coating over the surface of the lead frame before package fabrication. The protective coating should meet the requirements of reliability such as good solderability and non-cracking.
The blanket frames are typically plated with a layer of nickel (Ni) over the surface of the lead frames to form a protective coating. Nickel plating was intended to serve as a barrier against the the diffusion of copper atoms and prohibit formation of copper oxides on the surface of the lead frame. Furthermore, a solder layer of palladium (Pd) was formed by deposited a palladium or palladium/nickel alloy on top of the nickel layer in order to enhance solderability of the outer lead, as disclosed in U.S. Pat. Nos. 5,436,082, 5,454,929, 5,486,721, European Patent Application No. 0335608 and European Patent Application No. 0384586.
Unfortunately, the nickel layer in thickness of less than 40 microinches contains pores through which migration and diffusion of copper to the surface of the lead frame takes place. However, the nickel layer in thickness of greater than 40 microinches tends to crack when the lead are eventually bent. An attempt to reduce effects of diffusion of copper through a less than 40 microinches thick nickel layer was made by successively deposited a nickel layer, a layer of palladium or soft gold (Au) strike, a layer of palladium-nickel alloy, a layer of palladium and a layer of gold as disclosed in U.S. Pat. No. 5,360,991. However, the manufacturing process of in succession from the five-layered-structure (Au/Pd/Pd--Ni/Ni or Au strike/Ni/lead frame) is quite complicated and hard processed.
In recent years, the price of palladium is almost as that of gold; therefore, the conventional pre-plated frame with a solder layer of palladium has a drawback in that the manufacturing cost thereof is high.